Electrolyte comprising non-ionic surfactant and lithium ion battery using the same

ABSTRACT

The present invention relates to an electrolyte comprising non-ionic surfactant and a lithium ion battery using the same, and more particularly, to a non-aqueous electrolyte for a lithium ion battery comprising a fluorine-based non-ionic surfactant. The lithium ion battery prepared according to the present invention uses an electrolyte comprising a fluorine-based non-ionic surfactant that is substituted with various functional groups at the end group as represented by Formula 1, which can improve an interfacial property between an electrolyte and electrodes and impedance properties and exhibits a high capacity and excellent charge/discharge properties.

BACKGROUND OF THE INVENTION

[0001] (a) Field of the Invention

[0002] The present invention relates to an electrolyte comprising anon-ionic surfactant and a lithium ion battery using the same, and moreparticularly, to a non-aqueous electrolyte for a lithium ion batterycomprising a fluorine-based non-ionic surfactant.

[0003] (b) Description of the Related Art

[0004] Ever since the commercialization of lithium ion liquid secondarybatteries by Sony Co., the lithium ion liquid secondary battery has beenused increasingly in portable computers, cellular phones, etc., insteadof the prior art lithium ion secondary batteries as a result of its highenergy density. The lithium ion liquid secondary battery comprises ananode including carbonaceous material as an anode active material and acathode including metal oxide of LiCoO₂, etc. as a cathode activematerial, and is prepared by intercalating a porous polyolefin-basedseparator between the anode and the cathode, then by injecting anon-aqueous electrolyte having a lithium salt of LiPF₆, etc. When thebattery charges, the lithium ions of the cathode active material arereleased and then inserted into the carbon layer of the anode. When thebattery discharges, the opposite occurs with the lithium ions of acarbon layer of an anode being released and then inserted into thecathode active material.

[0005] The non-aqueous electrolyte plays a mediating role moving thelithium ions between the anode and the cathode. The electrolyte shouldbe stable within the scope of the operation voltage of the battery, andbe able to transfer the ions sufficiently at a fast velocity. As anelectrolyte, U.S. Pat. Nos. 5,521,027 and 5,525,443 discloses anadmixture electrolyte of a linear carbonate and cyclic carbonate. Thecyclic carbonate has a large polarity and thus is sufficiently capableof dissociating lithium, but has high viscosity. Therefore, in thesepatents, mixing linear carbonate with a low polarity and a low viscosityreduces the viscosity of the electrolyte comprising the cycliccarbonate. The different linear carbonates include dimethyl carbonate(DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. Thedifferent cyclic carbonates include ethylene carbonate (EC), propylenecarbonate (PC), vinylene carbonate (VC), butylene carbonate (BC), etc.The use of cyclic carbonates is necessary when it is desired to obtain ahigh capacity, good temperature properties, and a safe batteryconfiguration, and particularly, when using electrolytes having a highviscosity such as EC, PC, etc.

[0006] The non-aqueous electrolyte decreases operational efficiencybecause it is slowly penetrated into the active material of electrodes,and thus the performance of the battery deteriorates by increasing animpedance thereof because a sufficient capacity of the battery cannot beutilized. Accordingly, in order to improve an interfacial propertybetween the non-aqueous electrolyte and electrodes, Japanese PatentPublication Hei 8-306386 discloses a method of adding an anionicsurfactant to an electrolyte and Japanese Patent Publication Hei 9-30651discloses a method of adding an anionic surfactant directly to theelectrode slurry. However, satisfactory results have not been obtainedwith these two methods. The surfactant that is contained in anelectrolyte should not affect the other properties of a battery, shouldbe stable in the operation voltage range of a battery and increase aninterfacial activity between electrodes and an electrolyte.

SUMMARY OF THE INVENTION

[0007] The present invention was made in consideration of the problemsof the prior art, and it is an object of the present invention toprovide an electrolyte additive comprising a fluorine-based non-ionicsurfactant, which can improve impedance properties of a battery byimproving an interfacial property between an electrolyte and electrodes,and which can be used in preparing a lithium ion secondary batteryhaving a high capacity and high efficiency.

[0008] It is another object of the present invention to provide alithium ion battery comprising the electrolyte additive.

[0009] In order to achieve these objects, the present invention providesan electrolyte comprising a fluorine-based non-ionic surfactantrepresented by the following Formula 1:

[0010] wherein, R is hydrogen, an acetyl group, a methyl group or abenzoyl group; and

[0011] m and n are integers from 2 to 20.

[0012] The present invention also provides a lithium ion batterycomprising:

[0013] a) graphitized carbon that can reversibly store and releaselithium as an anode active material;

[0014] b) lithium-containing transition metal oxide that can reversiblystore and release lithium as a cathode active material;

[0015] c) a porous separator; and

[0016] d) an electrolyte comprising:

[0017] i) a lithium salt;

[0018] ii) an electrolyte compound; and

[0019] iii) a fluorine-based non-ionic surfactant represented by Formula1.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a graph comparing impedance properties of lithium ionbatteries of the present invention (Examples 1 to 6) and of generalbatteries;

[0021]FIG. 2 is a graph comparing impedance properties of lithium ionbatteries of the present invention (Examples 6 to 12) and of generalbatteries; and

[0022]FIG. 3 is a graph comparing impedance properties of lithium ionbatteries of the present invention (Examples 13 to 14) and of generalbatteries.

DETAILED DESCRIPTION AND THE PREFERRED EMBODIMENTS

[0023] The present invention will now be explained in more detail.

[0024] The present invention relates to an electrolyte comprising afluorine-based non-ionic surfactant represented by Formula 1 above. Inaddition, the present invention provides a lithium ion secondary batterycomprising an anode including graphitized carbon, a cathode includinglithium-containing transition metal oxide, a porous separator and anelectrolyte containing a fluorine-based non-ionic surfactant representedby Formula 1, in which the secondary battery has a large capacity andimproved impedance properties.

[0025] Generally, EC, which is a cyclic carbonate used for a batteryemploying a graphitized carbon anode, has a high solubility for lithiumsalts and a high ion conductivity. However, when EC is used in excess,it rapidly decreases low temperature properties of an electrolytebecause it has a melting point that is higher than room temperature. Inorder to solve this problem, 2-ingredient electrolytes containing linearcarbonates having a low melting point and a low viscosity have generallybeen used. Nevertheless, such an electrolyte increases impedance todeteriorate battery performance during high-speed charging/dischargingbecause it takes a long time for the electrolyte to penetrate into anactive material of electrodes when injected into a battery, and asufficient capacity cannot be obtained from the active material evenwith a long charge time.

[0026] The present invention can decrease an impedance of the whole cellby adding a fluorine-based non-ionic surfactant represented by Formula 1to an electrolyte such that the easy penetration of an electrolyte intoelectrodes occurs to decrease interfacial resistance. The surfactantadded should not affect the other properties of the battery and shouldbe stable in the operation voltage range of the battery. The surfactantrepresented by Formula 1 used in the present invention is a highmolecular compound substituted at each hydroxy group of its end groupwith acetyl, methyl or a benzoyl group by a common organic syntheticprocess in order to eliminate reactivity with an electrolyte. Forexample, MEGAFAC F-142D (n=10), F-144D (n=20) and F-142P (n=10, highpurity) purchased from DIANIPPON INK & CHEMICALS Company are substitutedat each hydroxy group of each end group with acetyl, methyl and benzoylgroups respectively to be used in the present invention.

[0027] The fluorine-based non-ionic surfactant represented by Formula 1has a hydrophobic group using a fluorocarbon ring instead of ahydrocarbon ring, which is resistant to heat and chemicals since it hasstrong carbon-fluorine bonds, and particularly which does not affect theother properties of the battery since it remains stable duringcharging/discharging of a battery. In addition, one end of thesurfactant that is covered with fluorocarbon has a much higherinterfacial tension at a surface of a solid electrode to improve theinterfacial properties between a solid electrode and an organic solvent.

[0028] Meanwhile, the present invention prepares a lithium ion secondarybattery having a large capacity and improved impedance properties usingan electrolyte comprising a fluorine-based non-ionic surfactantrepresented by Formula 1.

[0029] The lithium ion battery of the present invention comprises agraphitized carbon that can reversibly store and release lithium as ananode active material; lithium-containing transition metal oxides thatcan reversibly store and release lithium as a cathode active material; aporous separator; and a non-aqueous electrolyte comprising a lithiumsalt, an electrolyte compound and a fluorine-based non-ionic surfactantrepresented by Formula 1.

[0030] The graphitized carbon has preferably an interplanar spacing ofd002 of 0.338 nm or less as measured by X-ray diffraction ofcarbonaceous material, and has a specific surface area of 10 m²/g orless as measured by the Brunauer-Emmett-Teller (BET) method.

[0031] The lithium-containing transition metal oxide is preferablyselected from the group consisting of LiCoO₂, LiNiO₂, LiMn₂O₄ andLiNi_(1−X)Co_(X)O₂ (0<x<1).

[0032] The lithium salt is preferably at least one selected from thegroup consisting of LiClO₄, LiCF₃SO₃, LiPF₆, LiBF₄, LiAsF₆ andLiN(CF₃SO₂)₂.

[0033] The electrolyte compound is preferably selected from the groupconsisting of ethylene carbonate, propylene carbonate, butylenecarbonate, vinylene carbonate, diethyl carbonate, dimethyl carbonate,ethyl methyl carbonate, gamma-butyrolactone (GBL), sulfolane, methylacetate, and methyl propionate.

[0034] Futhermore, the contents of the fluorine-based non-ionicsurfactant represented by Formula 1 is preferably 0.01 to 1 wt % of theelectrolyte.

[0035] The battery using the components as mentioned above uses, forexample, an anode comprising carbon active material and a polyvinylidenedifluoride binder, and a cathode comprising lithium transition metaloxide active material, conductive carbon and a polyvinylidene difluoridebinder to realize a lithium ion battery.

[0036] As mentioned above, the present invention prepares a lithium ionbattery using a fluorine-based ethylene oxide surfactant represented byFormula 1, thereby enabling the sufficient penetration of an electrolyteinto the active material of a battery to decrease the impedance in thebattery. This increases the capacity of the active material andcharge/discharge efficiency.

[0037] Hereinafter, the present invention is described more in detailthrough EXAMPLES and COMPARATIVE EXAMPLES. However, the followingEXAMPLES are presented to enable better understanding of the presentinvention, and the present invention is not limited to the followingEXAMPLES.

EXAMPLES Preparation Example 1

[0038] (Synthesis of a fluorine-based ethylene oxide resin surfactantF-142d-Ac substituted with acetate at the end)

[0039] F-142d (average n=10, mean molecular weight=1379, 2.50 g) andtriethylamine (0.44 g) were dissolved in dichloromethane (25 mL), andacetyl chloride (0.57 g) was slowly added thereto at 0° C. undernitrogen atmosphere. After elevating the temperature of the mixture toroom temperature, the mixture was stirred for 15 hours. After thereaction was completed, the produced solids were filtered and thesolvent was distilled under reduced pressure. Diethyl ether (25 mL) wasadded thereto to further produce solids not produced in dichloromethane.The solids were filtered and removed, and diethyl ether was distilledunder reduced pressure and removed to obtain a liquid product. An acetylgroup of the product was identified by the peak near 4.2 ppm using aBruker 300 MHz NMR.

Preparation Example 2

[0040] (Synthesis of a fluorine-based ethylene oxide resin surfactantF-144d-Ac substituted with acetyl at the end)

[0041] F-144d (average n=20, mean molecular weight=2260, 2.50 g) andtriethylamine (0.27 g) were dissolved in dichloromethane (25 mL) andacetyl chloride (0.57 g) was slowly added thereto at 0° C. undernitrogen atmosphere. After elevating the temperature of the mixture toroom temperature, the mixture was stirred for 15 hours. After thereaction was completed, the produced solids were filtered and thesolvent was distilled under reduced pressure. Diethyl ether (25 mL) wasadded thereto to further produce solids not produced in dichloromethane.The solids were filtered and removed, and diethyl ether was distilledunder reduced pressure and removed to obtain a liquid product.

Preparation Example 3

[0042] (Synthesis of a fluorine-based ethylene oxide resin surfactantF-142d-Me substituted with a methyl group at the end)

[0043] Potassium hydroxide powder (85%, 0.72 g) was slowly added to asolution of F-1 42d (average n=10, mean molecular weight=1379, 2.5 g)and 1,4-dioxane (5 mL) while stirring in a bath at 65° C. Dimethylsulfate (0.35 mL) was added thereto at a rate of 3 drops per 5 minutes.The mixture was further stirred in a bath at 65° C. for 3 hours.

[0044] After the reaction was completed, the produced solids werefiltered and the solvent was distilled under reduced pressure. Layerswere separated using dichloromethane (25 mL) and water (25 mL), theorganic layer was taken and dried using anhydrous MgSO₄, and the organicsolvent was distilled under reduced pressure to obtain a liquid product.A methyl group of the product was identified by the peak near 3.38 ppmusing a Bruker 300 MHz NMR.

Preparation Example 4

[0045] (Synthesis of a fluorine-based ethylene oxide resin surfactantF-1 44d-Me substituted with a methyl group at the end)

[0046] Potassium hydroxide powder (85%, 0.44 g) was slowly added to asolution of F-144d (average n=20, mean molecular weight=2260, 2.5 g) and1,4-dioxane (5 mL) while stirring in a bath at 65° C. Dimethyl sulfate(0.21 mL) was added thereto at a rate of 3 drops per 5 minutes. Themixture was further stirred in a bath at 65° C. for 3 hours.

[0047] After the reaction was completed, the produced solids werefiltered and the solvent was distilled under reduced pressure. Layerswere separated using dichloromethane (25 mL) and water (25 mL), theorganic layer was taken and dried using anhydrous MgSO₄, and the organicsolvent was distilled under reduced pressure to obtain a liquid product.

Preparation Example 5

[0048] (Synthesis of a fluorine-based ethylene oxide resin surfactantF-142d-Bz substituted with a benzoyl group at the end)

[0049] F-142d (average n=10, mean molecular weight=1379, 2.5 g) andtriethylamine (0.44 g) were dissolved in dichloromethane (25 mL) andbenzoyl chloride (0.84 mL) was slowly added thereto at 0° C. undernitrogen atmosphere. After elevating the temperature of the mixture toroom temperature, the mixture was stirred for 15 hours.

[0050] After the reaction was completed, the produced solids werefiltered and the solvent was distilled under reduced pressure. Diethylether (25 mL) was added thereto and triethylamine (0.44 g) was furtheradded, and the mixture was stirred for 30 minutes. Further producedsolids not produced in dichloromethane and benzoyl chloridetrimethyamine salt were filtered and removed, then diethyl ether wasdistilled under reduced pressure and removed to obtain a liquid product.A benzoyl group of the product was identified by the peak near 7.46,7.57, and 8.08 ppm using a Bruker 300 MHz NMR.

Preparation Example 6

[0051] (Synthesis of a fluorine-based ethylene oxide resin surfactantF-144d-Bz substituted with a benzoyl group at the end)

[0052] F-1 44d (average n=20, mean molecular weight=2260, 2.5 g) andtriethylamine (0.27 g) were dissolved in dichloromethane (25 mL) andbezoyl chloride (0.51 mL) was slowly added thereto at 0° C. undernitrogen atmosphere. After elevating to the temperature of the mixtureto room temperature, the mixture was stirred for 15 hours.

[0053] After the reaction was completed, the produced solids werefiltered and the solvent was distilled under reduced pressure. Diethylether (25 mL) was added thereto and triethylamine (0.44 g) was alsofurther added, and the mixture was stirred for 30 minutes. Furtherproduced solids not produced in dichloromethane and benzoyl chloridetrimethyamine salt were filtered and removed, and diethyl ether wasdistilled under reduced pressure and removed to obtain a liquid product.

Preparation Examples 7-12

[0054] (Preparation of an electrolyte)

[0055] Each 0.1 wt % of the fluorine-based non-ionic surfactant preparedaccording to Preparation Examples 1 to 6 was added to a 1M LiPF₆solution with a composition of EC:EMC=1:1 using F-EC and F-EMC purchasedfrom Mitsubishi Chem. Company in a globe box to prepare electrolytes ofPreparation Examples 7 to 12.

Preparation Examples 13-18

[0056] (Preparation of an electrolyte)

[0057] Except for changing the amount of the added fluorine-basednon-ionic surfactants from 0.1 wt % to 0.01 wt %, electrolytes ofPreparation Examples 13 to 18 were prepared by the same method as inPreparation Examples 7 to 12.

Comparative Preparation Example 1

[0058] (Preparation of an electrolyte)

[0059] A 1M LiPF₆ electrolyte with a composition of EC:EMC=1:1 wasprepared using F-EC and E-EMC purchased from Mitsubishi Chem. Company ina globe box.

Examples 1-12

[0060] (Preparation of a Lithium Ion Battery)

[0061] 93% of carbon active material (MCMB-10-28 from Osaka Gas) and 7%of polyvinylidene difluoride (PVDF, Kynar 761 product from Elf AtochemCompany) were mixed in a mixer (Ika Company) for 2 hours usingpyrrolidone (NMP) as a solvent, and the mixture was coated on a copperfoil collector and dried at 130° C. to prepare an anode. 91% of LiCoO₂,3% of PVDF (Kynar 761) and 6% of conductive carbon (KS-6 from LonzaCompany) were mixed in a mixer (Ika Company) for 2 hours usingN-methyl-2-pyrrolidone (NMP), and the mixture was coated on an aluminumfoil collector and dried at 130° C. to prepare a cathode. Celgard 2400(from Hoechst Celanese Company) was put between the prepared anode andcathode for use as a separator to fabricate a coin-type battery, and theelectrolytes prepared in Preparation Examples 7 to 18 were respectivelyinjected therein to prepare lithium ion batteries of Examples 1 to 12.The batteries were charged to 4.2 V and discharged to 3V to perform acharge/discharge test. The results of the test are summarized in Table1.

Comparative Example 1

[0062] (Preparation of a Lithium Ion Battery)

[0063] Except for the injection of the electrolyte prepared inComparative Preparation Example 1, a lithium ion battery was prepared bythe same method as in Example 1. A charge/discharge test was performedby the same method as in Example 1, the results of which are presentedin Table 1. TABLE 1 Initial Capacity of Initial Charge Surfactant (wt %)Battery (mAh) Efficiency Comparative 3.75 87.62 Example 1 Example 1142d-Ac 0.1 3.81 89.81 Example 2 144d-Ac 0.1 3.80 89.38 Example 3142d-Me 0.1 3.82 88.25 Example 4 144d-Me 0.1 3.84 88.96 Example 5142d-Bz 0.1 3.83 88.75 Example 6 144d-Bz 0.1 3.82 88.75 Example 7142d-Ac 0.01 3.80 88.97 Example 8 144d-Ac 0.01 3.78 89.62 Example 9142d-Me 0.01 3.89 89.14 Example 10 144d-Me 0.01 3.78 86.61 Example 11142d-Bz 0.01 3.72 87.35 Example 12 144d-Bz 0.01 3.79 88.59

[0064] [Experiment 1]

[0065] (Impedance Properties)

[0066] The batteries prepared in Examples 1 to 12 and ComparativeExample 1 were charged to 4.2 V and discharged to 3.0 V, then after thisprocess was repeated, the batteries were charged to 4.2 V again and animpedance of the batteries was measured. The impedance was measured byscanning from 1 MHz to 1 mHz using a Potentiostat/Galvanostat, Model273A from EG & G PRINCETON APPLIED RESEARCH Company and the SI 1260Impedance/Gain-phase analyzer from Solatron Instruments Company. Theresults of the measurements are presented in FIGS. 1 and 2.

Preparation Example 19

[0067] (Synthesis of a fluorine-based ethylene oxide resin surfactantF-142P-Ac substituted with acetate at the end)

[0068] An acetate-substituted fluorine-based ethylene oxide resinsurfactant having a higher purity than 142d-Ac was obtained usingMEGAFAC F-142P (n=10) from DIANIPPON INK & CHEMICALS Company by the samemethod as in Preparation Example 1.

Preparation Example 20

[0069] (Preparation of an Electrolyte)

[0070] 0.1 wt % of the surfactant synthesized in Preparation Example 19was added to a 1M LiPF₆ solution with a composition of EC:EMC=1:1 usingF-EC and F-EMC purchased from Mitsubishi Chem. Company in a globe box toprepare an electrolyte.

Preparation Example 21

[0071] (Preparation of an Electrolyte)

[0072] 0.01 wt % of the surfactant synthesized in Preparation Example 19was added to a 1M LiPF₆ solution with a composition of EC:EMC=1:1 usingF-EC and F-EMC purchased from Mitsubishi Chem. Company in a globe box toprepare an electrolyte.

Examples 13-14

[0073] (Preparation of Lithium Ion Battery)

[0074] Lithium ion batteries of Examples 13 to 14 were prepared usingthe electrolytes of Preparation Examples 20 and 21, respectively, by thesame method as in Example 1.

[0075] [Experiment 2]

[0076] (Battery Performance Test)

[0077] An initial capacity, an initial charge efficiency and impedanceproperties of the batteries prepared in Examples 13 and 14 were measuredby the same method as in Examples 1 to 12. The results are presented inTable 2 and FIG. 3 TABLE 2 Initial Battery Initial Charge Surfactant (wt%) Capacity (mAh) Efficiency Comparative — — 3.75 87.62 Example 1Example 13 142P-Ac 0.1  3.80 88.31 Example 14 144P-Ac 0.01 3.79 88.40

[0078] As explained above, the lithium ion battery prepared according tothe present invention uses an electrolyte comprising a fluorine-basednon-ionic surfactant substituted with various functional groups at theend as represented by Formula 1, which improves an interfacial propertybetween an electrolyte and electrodes and impedance properties, andexhibits a high capacity and excellent charge/discharge properties.

What is claimed is:
 1. An electrolyte comprising a fluorine-basednon-ionic surfactant represented by Formula 1: [Formula 1]

wherein, R is hydrogen, an acetyl group, a methyl group or a benzoylgroup; and m and n are integers from 2 to
 20. 2. A lithium ion batterycomprising: a) graphitized carbon that can reversibly store and releaselithium as an anode active material; b) lithium-containing transitionmetal oxide that can reversibly store and release lithium as a cathodeactive material; c) a porous separator; and d) an electrolyte comprisingi) a lithium salt; ii) an electrolyte compound; and iii) afluorine-based non-ionic surfactant represented by Formula 1: [Formula1]

wherein, R is hydrogen, an acetyl group, a methyl group or a benzoylgroup; and m and n are integers from 2 to
 20. 3. The lithium ion batteryaccording to claim 2, wherein the a) graphitized carbon has aninterplanar spacing of d002 of 0.338 nm or less as measured by X-raydiffraction of carbonaceous material, and has a specific surface area of10 m²/g or less as measured by the Brunauer-Emmett-Teller (BET) method.4. The lithium ion battery according to claim 2, wherein the b)lithium-containing transition oxide is selected from the groupconsisting of LiCoO₂, LiNiO₂, LiMn₂O₄, and LiNi_(1−X)Co_(X)O₂(0<x<1). 5.The lithium ion battery according to claim 2, wherein the c) i) lithiumsalt is selected from the group consisting of LiClO₄, LiCF₃SO₃, LiPF₆,LiBF₄, LiAsF₆ and LiN(CF₃SO₂)₂.
 6. The lithium ion battery according toclaim 2, wherein the c) ii) electrolyte compound is at least oneselected from the group consisting of ethylene carbonate, propylenecarbonate, butylene carbonate, vinylene carbonate, diethyl carbonate,dimethyl carbonate, ethyl methyl carbonate, gamma-butyrolactone (GBL),sulfolane, methyl acetate, and methyl propionate.
 7. The lithium ionbattery according to claim 2, wherein the content of the c) iii)fluorine-based non-ionic surfactant represented by Formula 1 is 0.01 to1 wt % of the electrolyte.